Acute Phase Proteins

The host defense mechanism initiates a series of well-coordinated systemic reactions in response to trauma, injury or infection. Collectively this is termed the acute phase response, which is a component of innate immunity. There are a number of physiological alterations during an acute phase response. These include fever, leukocytosis, and a reduction in plasma concentrations of minerals, such as zinc and iron. Another component of the acute phase response is the induction of acute phase proteins (APPs), which is regulated by cytokines.

Cytokines are soluble messenger molecules that mediate signals between the site of inflammation and the hepatocytes that synthesize the APPs. Some of the pro-inflammatory cytokines that regulate the synthesis and release of APPs include TNF-α and IL such as IL-1 and IL-6. These cytokines are produced primarily by activated monocytes. However, there are a number of other cells that produce cytokines. Release of IL-1 and TNF-α cause a primary signal that results in the release of IL-6. Together these pro-inflammatory cytokines induce the synthesis and release of APPs. Acute phase proteins are categorized by the direction of changes in their concentrations in response to cytokines. Positive acute phase proteins such as haptoglobin (Hpt), and serum amyloid A, are significantly increased in plasma and milk when an acute phase response is initiated, and negative acute phase proteins, such as albumin, transthyretin (TTR), and retinol binding protein (RBP), are significantly decreased in plasma when an acute phase response is initiated. The APPs are used as indicators of infection and inflammation.

Haptoglobin

Haptoglobin is a 125 KDa protein that is thought to bind strongly to free hemoglobin, which results in limiting iron-dependent bacterial growth and is induced by proinflammatory cytokines. Haptoglobin is synthesized in the liver, lung, adipocytes, and mammary gland, and represents a sensitive positive APP in cattle. Concentrations of Hpt can increase by more than 100-fold during acute inflammation. Several studies have also proposed that Hpt may be involved in development of fatty liver syndrome as its concentration is increased in cows with liver fat infiltration. In addition, several studies cited by Petersen et al. have demonstrated a relationship between mastitis and plasma Hpt. Cows with mastitis had greater plasma and milk concentrations of Hpt as compared to cows without mastitis. Serum Hpt concentrations peaked 3 d after onset of a severe intramammary coliform infection. Another positive APP, α-1 acid-glycoprotein, peaked 9 d after appearance of mastitis symptoms. This indicates that APPs have individual temporal patterns in response to inflammation. This feature can be used to differentiate between a rapid and slow acute phase responses. Recently, Eckersall et al. reported that cows challenged with Staph, aureus had a peak Hpt concentration in milk approximately 15 hr post-challenge whereas serum Hpt peaked approximately 24 hr post-challenge. This observation suggests that milk Hpt concentration may be a useful marker for IMI detection during the early stages of the inflammation.

Haptoglobin (Hpt)

In some studies no relationship between severity of mastitis and plasma concentrations of Hpt was detected. However, Jacobson et al. demonstrated that as the magnitude of E. coli lipopolysaccharide (LPS) challenge increased, there was a dosedependent increase in serum Hpt concentrations, indicating a Gram negative pathogen-specific response by Hpt. They demonstrated that serum concentration of albumin, a negative acute phase protein, was decreased as a result of E. coli LPS challenge. In one study, Humblet et al. also reported a significant reduction in serum albumin in calves with inflammation caused by bronchopneumonia as compared to calves that had recovered from the disease. Exposure to Pasteurellahaemolytica also caused significant reduction in plasma albumin whereas plasma Hpt concentrations were increased in Holstein calves. Responses in opposite direction by albumin and Hpt have been further supported by in vitro studies, where bovine hepatocytes showed reduced albumin and increased Hpt concentrations in response to bovine IL-6. Taken together, synthesis of plasma APPs are induced or suppressed by pro-inflammatory cytokines in response to inflammations and infections, such as an IMI. Information is currently limited on alterations in plasma Hpt and albumin concentrations in relation to naturally occurring new IMI during the periparturient period, and whether breeds differ in their response to occurrence of a new IMI.

Retinol binding-protein and transthyretin

Retinol is primarily transported by RBP from the liver to target organs. This transport protein is relatively small, made of a single polypeptide chain, has one binding site for retinol in the all-trans form and is primarily synthesized in the rough endoplasmic reticulum of the liver. Both vitamin A deficiency and toxicity can significantly reduce serum concentrations of RBP. Approximately 95% of plasma RBP is bound to TTR, which is a thyroid hormone transporter. Ong et al. have shown that RBP and TTR are synthesized and secreted by retinal pigment epithelium as an extra-hepatic source. Others have shown that RBP is also present in bovine placental membranes and uterine tissues. Retinol binding protein is a negative APP and decreases in plasma in response to infection, and may function to limit retinol availability for microbial growth. Decreased plasma RBP is due to the induction of the transcription factor NF-IL6 by pro-inflammatory cytokines such as IL-1, IL-6, and TNF-a. This induction results in down-regulation of the hepatic synthesis of proteins such as albumin and TTR. Plasma RBP is bound to plasma TTR and thus reduction in TTR results in reduction of plasma RBP. There is evidence that RBP plays an important role in immune function. Plasma concentrations of retinol and RBP were reduced in rats challenged with LPS from Pseudomonas aeruginosa. Rosales et al. suggested that secretion of the retinol-RBP complex was decreased with inflammation by LPS administration.

Plasma concentration of TTR was reduced in lactating cows as compared to dry cows. This finding was related to functional and morphological changes in the liver that are associated with increased concentrations of BHB. A negative correlation was also observed between plasma concentrations of BHB and TTR.